In certain types of radar systems a T-R (transmit-receive) switch is employed to block microwave signals above a given power level, while passing signals below the given power level, thereby preventing excessive bursts of power from destroying the receiving equipment. One type of device which has been employed for this purpose is a gas discharge switching tube. In the operation of such tubes a substantial recovery time during which signals cannot be received exists after each discharge; hence the pulse repetition rate of systems incorporating this type of switch is severely limited.
Another type of device which has been employed for microwave power switching utilizes a secondary electron resonance phenomenon termed "multipacting". In multipactor devices a radio frequency electric field is applied to an evacuated chamber including a pair of spaced opposing surfaces each having a secondary electron emission coefficient greater than unity. If the radio frequency electric field is of sufficient amplitude and if the frequency of the electric field is properly related to the surface spacing, electrons will be emitted from one surface and accelerated toward the opposite surface where they will arrive when the electric field reverses its polarity. Secondary electrons will be emitted from the opposite surface, and if the yield of secondary electrons is greater than one, more electrons will be emitted from this surface than impinged upon it. Since the electric field reverses its polarity as the secondary electrons are emitted, these secondary electrons will be accelerated back to the first surface from which they will release new secondary electrons at the same time the electric field reverses its polarity. Thus, the process continues as electrons are accelerated back and forth between the surfaces in synchronism with the alternating electric field. The net result is to establish multipactor action, i.e., electron multiplication in a synchronous alternating electric field between secondary electron emissive surfaces.
The aforedescribed phenomenon may be utilized to provide radio frequency power switching because when the input power to a multipactor switch is greater than the level required to sustain multipactor action, radio frequency power is absorbed by the electrons and is dissipated when these electrons strike the secondary electron emissive surfaces, thereby limiting the power transmitted through the switch to a predetermined lower level.
In previous multipactor switches, a thermionic electron emissive electrode has been employed to provide the multipactor region of the switch with electrons and thereby ensure a rapid commencement of multipactor action in response to an input signal above a given power level. Such an arrangement requires a power supply to provide heating current sufficient to cause thermionic emission of electrons from a cathode or filament, and when the cathode is located externally of the multipactor chamber, a second power supply is required to bias the cathode sufficiently negatively with respect to the multipactor chamber so that the emitted electrons are accelerated into the multipactor region. Further details concerning the aforedescribed multipactor switching devices may be found in U.S. Pat. Nos. 2,674,694 to W. R. Baker, 3,354,349 to K. L. Horn, and 4,199,738 to T. P. Carlisle et al.